1. Field
This invention pertains to devices and methods for performing optical measurements using a plurality of light spots, and more particularly, to a method of locating valid light spots for use in making optical measurements by a measurement instrument, and a measurement instrument employing such a method of locating the valid light spots that are employed in its measurements.
2. Description
There are some instruments which employ light spots to make optical measurements. Well-known examples include instruments which employ a Shack-Hartmann wave front sensor.
FIG. 1 illustrates some principal elements of a basic configuration of a Shack-Hartmann wavefront sensor 100. Shack-Hartmann wavefront sensor 100 comprises a micro-optic lenslet array 110 and an optical detector 120. Typically, the optical detector 120 comprises a detector array or pixel array, for example, a charge-coupled device (CCD) camera or CMOS array.
The lenslets of lenslet array 110 dissect an incoming wavefront and create a pattern of light spots 130 that fall onto optical detector 120. In one typical embodiment, lenslet array 110 includes hundreds or thousands of lenslets, each on the size scale of a hundred microns. Meanwhile, optical detector 120 typically comprises many pixels (e.g., about 400 pixels) for each lenslet in lenslet array 110. Typically Shack-Hartmann sensor 100 is assembled such that the pixel array 120 lies in the focal plane of lenslet array 110.
Shack-Hartmann wavefront sensor 100 uses the fact that light travels in a straight line to measure the wavefront of light. By sensing the locations of light spots 130, the propagation vector of the sampled light can be calculated for each lenslet of lenslet array 110. The wavefront of the received light can be reconstructed from these vectors.
To better understand one or more aspects of this invention, it is worthwhile to discuss the operation of Shack-Hartmann wavefront sensor 100 in more detail. However, embodiments of the present invention extend to other types of optical measurement devices and systems such as topographers. In certain embodiments of the present invention, a system includes two or more optical measurement devices, for example, a combined system including both a wavefront sensor and a topographer.
In the case of the wavefront sensor 100, some optical system is employed to deliver a wavefront onto the lenslet array 110, which samples the wavefront over the tiny regions of each lenslet. Beneficially, the lenslets are much smaller than the wavefront variation. For the purposes of this discussion, we define “isoplanatic” as the condition where the wavefront is well approximated by a plane wave over an area the size of a lenslet. In that case, the wavefront is preferably isoplanatic over the sampled region. When optical detector 120—hereafter referred to more specifically as “detector array 120”—is in the focal plane of lenslet array 110, each lenslet will create a light spot on detector array 120. The locations of these light spots reveal the average of the wavefront slopes across each region. That is, the shift in the location of a light spot is proportional to the average of the slope of the wavefront over the region sampled by the corresponding lenslet that produced the light spot. Software may compute the shift in each light spot.
In a typical operation, a reference beam (e.g., a plane wave) is first imaged onto lenslet array 110 and the locations of the resultant light spots (“reference locations”) on detector array 120 is recorded. Then, a wavefront of interest is imaged onto lenslet array 110, and the locations of the light spots on detector array 120 produced by the wavefront of interest are recorded and compared against the reference locations.
FIGS. 2A-2E illustrate an idealized example of this process where a reference beam and a wavefront of interest are imaged onto a detector array of a wavefront sensor. This idealization shows the process of measuring a spherical wave with a wavefront sensor with just 16 lenslets.
The first step, as represented by the FIGS. 2A-2C, is to measure a plane wave and measure the corresponding series of light spot locations 210 which are used as reference locations 220. For the plane wave, each lenslet of lenslet array 110 produces one light spot on a location 210 within a corresponding Area of Interest (AOI) of pixel array 120 that lies beneath that lenslet.
The next step, as depicted in FIGS. 2D-2E, is to introduce a wavefront of interest and determine the shifts in the locations 240 of the light spots 230 from their reference locations 220.
Where the isoplanatic condition is satisfied and where the light spot shift is consistent with the small angle approximation of Fresnel, then the light spot shift is exactly proportional to the average of the wavefront slope over the lenslet. The incident wavefront is then reconstructed from the measurements of the average of the slopes for the hundreds or thousands of lenslets in the lenslet array.
Further details regarding the construction and operation of a Shack-Hartmann wavefront sensor and a system for measuring aberrations in an eye using the Shack-Hartman wavefront sensor are described in U.S. Pat. No. 7,122,774, issued on 17 Oct. 2006 to Daniel R. Neal et al., the entirety of which is hereby incorporated by reference for all purposes as if fully set forth herein.
One important application for Shack-Hartmann wavefront sensors is in the field of ophthalmic aberrometry. In common practice, a measurement instrument employing a Shack-Hartmann wavefront sensor injects near infrared light into a patient's eye which focuses on the retina and scatters back toward the instrument. This light is imaged onto the Shack-Hartmann lenslet array, and each lenslet in the lenslet array focuses the local portion of the incident light it intercepts onto the detector array, as described above. Data pertaining to the locations of the light spots is used to derive slope information using a least squares fit method, and thereby to construct the wavefront of the received light.
The nominally rectilinear array of light spots is produced by a rectilinear lenslet array. The detailed analysis of the locations of these light spots relative to their reference locations (i.e., the locations that result when a true plane wave is applied to the lenslet array) yields the local gradient of the incident wavefront. The overall area in which focal spots are present is determined by the patient's pupil, and analysis of this illuminated area yields the location size and shape of the pupil.
The application of Shack-Hartmann wavefront sensors to ophthalmic aberrometry has been a success.
However, the quality of the fit data, usually evaluated using Zernike coefficients, is affected by the quality of the light spot location data, and therefore it is important to ensure that the data quality is adequate to the measurement accuracy and precision requirements.
In particular, if the wavefront is not isoplanatic, the quality of the light spots erodes rapidly and it becomes more difficult to determine the locations of the light spots. More specifically, measuring highly aberrated light beams can lead to focal spot crossover such that light spot from a particular lenslets end up in locations on the pixel array that lie under neighboring lenslets, or end up in irregular locations not defined by a grid. As explained above with respect to FIGS. 2A-C, in a traditional system the first step to locating the light spots is to search in a predefined Area of Interest (AOI). However with a highly aberrated beam, there could be more than one light spot in an AOI, or an AOI for a particular pixel may include part of a light spot for a neighboring pixel. This will lead to errors in these traditional light spot location algorithms.
On method of dealing with this problem is the inclusion of a dynamic range limiting aperture in the measurement instrument to prevent to prevent light spots from appearing outside their AOIs on the detector array, as described for example in U.S. Pat. No. 6,550,917 issued on 22 Apr. 2003 to Daniel R. Neal et al., which is hereby incorporated by reference for all purposes as if fully set forth herein. The dynamic range limiting aperture clips portions of the light beam that impinge on the detector array above a certain angle. Accordingly, the dynamic range limiting aperture limits the dynamic range of the measurement instrument. So other methods of handling highly aberrated wavefronts are desired.
In addition to the problem of determining the locations of light spots for a highly aberrated light beam, in some cases lenslet array 110 may not be perfectly aligned with the pixels of detector array 120. That is, there may be a translational offset and/or a rotation angle between lenslet array 110 and detector array 120 that may complicate the wavefront analysis.
Additionally, there is a problem of determining what constitutes a valid light spot on the detector array.
FIG. 3 shows a typical raw image from a wavefront sensor. Some important error sources are illustrated in FIG. 3 and will be described below.
The incident near infrared beam not only scatters from a patient's retina, but also reflects directly from the patient's cornea. The use of a Range Limiting Aperture (RLA) in the measurement instrument, as described above, can significantly reduce the intensity of the reflected light. However, this so-called “corneal reflex” is generally orders of magnitude brighter than the desired retinally scattered light, and—beneficially—may be excluded from the wavefront calculations. Indeed, as is illustrated by reference numeral 310 in FIG. 3, the reflex can affect a neighborhood of nearby focal spots by introducing stray light that can alter the true light spot location or mask the light spot entirely. The location and intensity of the corneal reflex is affected by corneal shape and the actual position of the patient's eye when the data is acquired. For these reasons, the qualification and/or exclusion of light spot data in and around the corneal reflex can be challenging.
The retinal scatter that is necessary for the aberrometer measurement is highly speckled because the retinal structure is quite rough compared to the wavelength of the probe beam. This leads to variability in the relative brightness of the focal spots. A measurement instrument may employ a broadband probe beam to reduce the speckle, but even so, the intensity of the light spots can vary by a factor of four in a normal clear eye. Additional variation can be introduced by cataracts, “floaters” and opaque regions in pathological crystalline lenses. Cataracts diffuse the incident and return beams causing both reduced spot intensity and broader light spots. As a result, as shown in FIG. 3, the raw image from the wavefront sensor may include dim light spots 320 and/or missing light spots 330.
Additional reflections of the probe beam may be produced by each surface in eyes implanted with intraocular lenses (IOLs). While similar to the corneal reflex phenomenon, multiple reflections are typically present in these patients and may be far from the optic axis of the wavefront sensor. Also, in subjects with diffractive IOLs, it is expected that one lenslet focal spot per diffraction order transmitted through the optical system may be present. In some cases this will lead to two or more focal spots that may or may not be spatially separated. Such focal spot distributions can lead to inaccurate identification of the light spot location and therefore inaccurate wavefront measurements.
Another source of error in wavefront measurements is tear film breakup. Tear film break up can affect the location and sharpness of the light spots in the vicinity of the breakup. Tear film breakup is correlated to delays in blinking the eye. Some measurement systems may be designed to operate rapidly and reduce tear film breakup effects by avoiding the need to keep the patient from blinking for long periods. Nevertheless, it is still possible that light spots are affected by tear film breakup. This can negatively impact the resultant wavefront measurements.
The spot location algorithms used with a typical wavefront measurement instrument are designed to work with data taken within the nominal linear range of the detector device (e.g., a CMOS detector). It is apparent that the light spot location information is compromised when the spot brightness is poor compared to stray light and camera noise.
Therefore, it would be desirable to provide a method of locating valid light spots for use in optical measurements performed by a measurement instrument, particularly when measuring highly aberrated wavefronts. It would also be desirable to provide a measurement instrument employing a method of locating valid light spots for use in its optical measurements, particularly when measuring highly aberrated wavefronts. It would further be desirable to provide such a method and instrument that qualify light spots to verify their validity as part of the light spot location process.
In one aspect of the invention, a method is provided for locating a set of valid light spots produced on a pixel array by a wavefront of interest. The method includes a first stage and a second stage. The first stage comprises selecting a pixel of the pixel array. The second stage comprises: determining whether a light intensity value for the selected pixel is greater than a threshold, and when the light intensity value for the selected pixel is determined to be greater than the threshold, determining whether the selected pixel belongs to a valid light spot. The second stage further includes, when the selected pixel is determined to belong to a valid light spot, saving light spot location data indicating a determined location for the valid light spot with respect to the pixel array, and masking out a group of pixels of the pixel array corresponding to a defined area at the determined location of the valid light spot such that the masked pixels are considered for a remainder of the method to have light intensity values less than the threshold. The method further includes repeating the first stage and the second stage for a plurality of pixels of the pixel array.
Qualifying the set of light spots includes, for each light spot in the group of light spots: calculating a first calculated location of the light spot using a first calculation algorithm; calculating a second calculated location of the light spot using a second calculation algorithm different from the first calculation algorithm; and when a difference between the first and second calculated locations for the light spot is greater than an agreement threshold, excluding the light spot from the qualified set of light spots. In addition, the light spots excluded from the qualified set may be excluded from being employed in determining the property of the object.
In another aspect of the invention, a device includes: a pixel array; a light spot generator adapted to receive light from an illuminated objected and to produce a group of light spots on the pixel array from the light received from the illuminated object; and a processor adapted to determine locations of the light spots on the pixel array by executing an algorithm. The algorithm comprises: a first stage and a second stage. The first stage comprises selecting a pixel of the pixel array. The second stage comprises: determining whether a light intensity value for the selected pixel is greater than a threshold, and when the light intensity value for the selected pixel is determined to be greater than the threshold, determining whether the selected pixel belongs to one of the light spots. The second stage further includes, when the selected pixel is determined to belong to one of the light spots, saving light spot location data indicating a determined location for the one light spot with respect to the pixel array, and masking out a group of pixels of the pixel array corresponding to a defined area at the determined location of the one light spot such that the masked pixels are considered for a remainder of the algorithm to have light intensity values less than the threshold. The algorithm further includes repeating the first stage and the second stage for a plurality of pixels of the pixel array.
In yet another aspect of the invention, a method comprises: sequentially processing pixels of an image detector, wherein processing a pixel includes determining whether a light intensity detected by the pixel is greater than a threshold; when the light intensity detected by the pixel is determined to be greater than the threshold, and determining whether the pixel belongs to a valid light spot for measuring a wavefront of interest. The method further includes, when the pixel is determined to belong to a valid light spot, saving data indicating a location for the valid light spot, and masking out a group of pixels of the image detector at the location of the valid light spot such that the masked pixels are considered to have a light intensity less than the threshold for a remainder of the sequential processing of the pixels.